In the past, numerous circuits have been devised which selectively changed the output level of a circuit between various levels.
Bradley, in U.S. Pat. No. 3,783,380, teaches a switching circuit for connecting an input either (a) directly to an output by shortcircuiting the input to the output or (b) through a resistor to the output, both terminals, of the resistor being permanently coupled to ground through respective resistors. While the reference discloses the selective varying of impedance between input and output, it does not disclose the fixing of the input to a constant level of voltage regardless of selected impedance and does not disclose the switching of impedances to effect a selectable (approximate) 6dB variation in the ratio of (a) the output voltage and (b) a reference level defined by a leveller circuit at the input end. Bradley, it is also noted, does not suggest how a single arrangement of circuit elements can be switched to effect ratio doubling while, at the same time, maintaining a constant voltage across the input thereto.
In U.S. Pat. No. 3,015,790, Eisaman discloses a circuit wherein the movement of ganged switches between two positions results in a step change in voltage. Therein, three resistors are placed in series across a battery; the output of the circuit being selectively taken across the first two or the second input to (a) define an ideal source input and (b) define a reference voltage against which the circuit output is compared is not considered by Eisaman. Also, selectively connecting an impedance either (a) between the input and an output circuit or (b) between the output circuit and ground is not suggested.
Further, in IEE Transactions on Instrumentation and Measurement, Volume IM-27, No. 1, March 1978, pp. 76-69, Somlo discusses a voltage doubling circuit for absolute calibration of attenuation measurement systems. Specifically, Somlo uses 3-dB waveguide couplers for providing two signal paths having identical phase and amplitude. Each path is provided with switching means for selectively including resistors connected to ground and disposed along such path. According to this design, however, the two paths must be isolated and maintain phase and amplitude identity which, Somlo indicates, cannot be achieved completely with his design. In addition, providing a constant voltage across the input to the circuit in Somlo regardless of whether or not a "voltage doubling" is performed, is not considered.
Further, it is noted none of the above patents or articles provide two impedances wherein either of which or both of which in parallel can be connected either in a series circuit between the input and output of a doubler circuit--with the other impedance, if any, being included in a shunt circuit to ground. Accordingly, the patents and articles do not teach a circuit having at its input what appears to the load to be a virtual ground (at a maintained constant voltage).